<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div{
        width: 200px;
        height: 200px;
        background-color: red;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    .box{
        width: 100px;
        height: 100px;
        background-color: blue;
    }
</style>
<body>
    <div>
        <div class="box">我是div</div>
    </div>
</body>
</html>